Technical Field
The present invention relates to a heat insulator. The invention particularly relates to a heat insulator including a porous sintered body formed of MgAl2O4 and having excellent heat-insulating properties in a temperature region of 1000° C. or more. That is, the present invention relates to a heat insulator including a spinel porous sintered body.
Description of the Related Art
As a material for a heat insulator whose increase in heat conductivity in a high temperature region of 1000° C. or more is suppressed, which also has excellent heat resistance, a magnesia spinel ceramic porous body has been attracting attention.
JP 2012-229139 A (Patent Document 1) and JP 2013-209278 A (Patent Document 2) disclose that when a spinel ceramic porous body has a predetermined pore size distribution, heat conduction and radiant heat transmission can be suppressed, and that such a porous body can thus be used as a heat insulator having excellent heat resistance even at a high temperature of 1000° C. or more.
However, although the spinel ceramic porous body described in Patent Documents 1 and 2 has low heat conductivity at 1000° C. or more, which is a higher temperature than before, and also has excellent heat resistance, because of its high porosity, strength has been insufficient.
In order to improve strength, generally, a technique of reducing porosity to increase bulk specific gravity is employed. However, in the heat insulator described in Patent Documents 1 and 2, the simple reduction of porosity results in an increase in heat conductivity and also an increase in bulk specific gravity. Therefore, the demand for a heat insulator that has low heat conductivity and also is lightweight and easy to handle has not been sufficiently met.
Meanwhile, in recent years, a heat insulator whose increase in heat conductivity is suppressed even in a high temperature region of 1000° C. or more, which also is lightweight and has high strength, tends to be required.
As an example of a lightweight and high-strength heat insulator, a composite material including a heat insulator formed of a porous body and a material containing fibers is known.
For example, JP 10-226582 A (Patent Document 3) describes the invention of a multilayer heat insulator that can be used in a temperature region of more than about 1500° C., is produced in a simple manner, and has excellent mechanical characteristics and heat resistance. The multilayer heat insulator includes the following three layers (A), (B), and (C): (A) a heat-resistant layer including 75 to 95 wt % mullite fibers and 5 to 25 wt % silica fibers, (B) an intermediate layer, and (C) a heat-insulating layer including 15 to 35 wt % mullite fibers and 65 to 85 wt % silica fibers, and also includes a vitreous boron compound that fixes the entanglement points of the fibers to form a three-dimensional network.
In the case where the spinel ceramic porous body described in Patent Documents 1 and 2 is applied to the invention described in Patent Document 3, when high strength is to be ensured while maintaining the excellent heat-insulating properties at high temperatures, the heat insulator becomes heavy in weight. In addition, the ease of handling, that is, handleability at the time of working, has also been insufficient. In addition, an increase in weight results in increased volumetric specific heat, whereby an increased amount of heat may be required to raise the temperature of the heat insulator.